close to that of a stable population which has been generated by many year's interactions of constant low fertility and constant low mortality with the originally rugged population. The reader may be quite impressed with that picture in which the accordion-shaped population contour remains quite tenaciously for so many years, nearly one century long. Anyhow, here population ageing will start getting stabilized as a result of population dynamics operated for many years.
Population ageing can be measured by various indices. The most popular one deals with the proportion of aged population which is aged 65 and over. If this proportion increases, it is called population ageing. If the proportion stays too small, however, say five percent or less, we do not usually call the population "aged" or "ageing". According to the United Nations' report published in 1956, the population is arbitrarily defined as "aged" when the percentage of old people aged 65 and over exceeds seven percent (United Nations, 1956). In view of the present levels of the population ageing in the developed countries, however, the figure of seven percent seems too small. In the present author's view, the thr~hold value of 10 percent seems more appropriate. As another index, a use is made of age dependency ratio for the elderly, that is the ratio of the elderly population over the working-age population of 15-64 or 20-64 years. The third often-used indicator is the elderly-children ratio, that is the ratio of the elderly over the children. Tables 1 and 2 show the trends in the age structure of population in Japan, one for the past and the other for future. By any measure, Japan is experiencing an increasingly pronounced and rapid process of population ageing.
Let us discuss and explain a little bit more of the trends in these indicators.
B. Proportion of the Aged
As already mentioned, the age composition of Japan has undergone a very sharp transformation, from a broad-based, youth-heavy population to a more urn-shaped, top-heavy ageing population, in a relatively short period of time. Table 1 shows the change in the age composition in terms of various indicators for the period from 1868 to 1995; Table 2 shows the projected transformation for the periods from 1995 to 2090 based on the population projections prepared recently in Japan in 1992 (Institute of Population Problems, 1992).
Columns (2) to (4) in these tables show percentages of population for
